The invention relates to an electrically insulating supporting structure being mechanically loadable in all directions and being capable of metallic bonding to electrically conductive components being electrically separated from one another.
Often such components must nevertheless be held together mechanically, and therefore a large number of electrically insulating supporting structures are known. In general, plastics or ceramic components are used for purposes of electrical insulation.
However, problems always occur whenever the electric insulations are at the same time exposed to considerable mechanical strains. For instance, ceramic structures can withstand loads but not major tensile forces. Moreover, if tensile forces are to be withstood, ceramic structures must be secured to the electrically conductive components, which again presents problems. Although metallized ceramics are known that can be brazed on, for instance, nevertheless once again the tensile strain that can be withstood is only that which the ceramic itself allows.
International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711 discloses an electrically heatable honeycomb body with internal support structures, in which individual components or partial regions or areas must be electrically insulated from one another. At the same time, extreme temperature strains and extreme mechanical loads because of thermal expansion also occur.
It is accordingly an object of the invention to provide an electrically heatable catalytic converter and an electrically conductive honeycomb body using the same, which overcome the herein afore-mentioned disadvantages of the hereto fore-known devices and methods of this general type, in which the supporting structure is capable of bearing high mechanical loads in all directions, and in particular tensile loads and wherein the supporting structure is particularly suitable for use in an electrically heatable honeycomb body, for which extreme loads are typical.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electrically insulating supporting structure being mechanically loadable in all directions for metallic bonding to electrically conductive components being electrically separated from one another, comprising a first metallic structure; a second metallic structure; and an electrically insulating, ceramic material being disposed between the first and second metallic structures and being three-dimensionally pressed-in from substantially all sides; the first and second metallic structures being shaped to exert pressure from substantially all sides onto the ceramic material, without touching one another.
A decisive factor for the present invention is that two metallic structures are disposed in such a way that while they do not touch each other, nevertheless they practically completely surround a ceramic layer located between them and do not allow it to escape in any direction. It is unimportant whether the ceramic layer is a powdered material or is a molded part. The structures must be shaped in such a way that neither a powder nor fragments of a molded part crumbling in response to mechanical loads can fall out. The outstanding insulating properties of ceramic powder are well known, for instance in the field of thermocouples and jacket measuring conductors. The ceramic and metal materials which are suitable for such purposes are also suitable for the present invention. It is important that the supporting structure have a capability of metallic bonding to components that are electrically separated from one another. The first metallic structure must be connectable to a first component, and the second metallic structure to a second component being electrically insulated from the first.
In accordance with another feature of the invention, a simple and especially highly suitable construction provides that the first metallic structure is a continuous wire or a continuous band, and the second structure includes individual portions that encompass the wire or the band, wherein ceramic material is pressed-in between the two metallic structures. In this way, one component can be joined metallically to the wire or band, while another component, being electrically insulated from it, can be connected to the outer portions. Regardless of the direction in which forces are exerted on the two components, the ceramic material between the two metallic structures is only under compressible strain, because it is enclosed on practically all sides. The load bearing capacity of the supporting structure is therefore determined by that of its metallic parts, not by that of the ceramic material.
In accordance with a further feature of the invention, the ceramic material includes individual ceramic beads or small tubes, or may be in powder form. Nor does it play any role if initially compact ceramic beads or small ceramic tubes should breakdown over time, under alternating thermal strains, into fragments or powder, since the fragments cannot be lost.
In accordance with an added feature of the invention, it is especially advantageous and suitable for larger-surface-area supporting structures if the first metallic structure is form-lockingly connected to the second metallic structure by one or two bulges having at least one bottleneck, for instance in the manner of pushbuttons, wherein ceramic material, especially a thin metal oxide film, is present between the structures. In the region of the bulging features and the bottleneck, the ceramic intermediate layer cannot escape, and just as in the examples described above, regardless of the direction of the forces acting on the structure, it is always subject to pressure only. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements.
In accordance with an additional feature of the invention, it is especially advantageous to initially join the first and second metallic structures form-lockingly to one another, and only after that to form an insulating layer between them. This can be performed by oxidation or chemical treatment of the surface, or by a special coating of one of the surfaces between the two structures.
With the objects of the invention in view, there is also provided a method for producing an electrically insulating supporting structure being mechanically loadable in all directions for metallic bonding to electrically conductive components being electrically separated from one another, which comprises pressing-in an electrically insulating ceramic material, preferably in powdered form, on substantially all sides between a first metallic structure and a second metallic structure; and shaping the first and second metallic structures to be both accessible, at least in partial regions, and to exert pressure from substantially all sides onto the ceramic material, without touching one another, for metallic bonding to further structures.
Once again it is important that both the first and the second metallic structures be accessible for metallic bonding to further structures, yet nevertheless not touch one another and between them press-in a ceramic material which is always subject to pressure, regardless of the direction of the forces occurring at the metallic structures.
In accordance with another mode of the invention, the first metallic structure is surrounded by a ceramic layer and a metal envelope, and the surrounding metal envelope is subdivided into a plurality of individual segments. It is important in this respect that the end of the individual segments of the outer metal structure not be simply open but rather be shaped on the inside in such a way that they press-in the ceramic material between them on practically all sides. For instance, a supporting structure according to the invention can be made from a conventional jacket conductor by constricting individual segments and then removing the constricted portions of the jacket (for instance mechanically by etching, or by melting).
With the objects of the invention in view, particularly for large-area supporting structures, there is additionally provided a method for producing an electrically insulating supporting structure being mechanically loadable in all directions for metallic bonding to electrically conductive components being electrically separated from one another, which comprises joining together a first metallic structure and a second metallic structure with three-dimensional form-locking connections by joint deformation; forming a surface or a surface layer being convertible chemically into an electric insulating layer on at least one of and between the first and second metallic structures; and subsequently converting the convertible surface or surface layer into an electrically insulating ceramic layer, in particular by oxidation, creating an insulating layer being pressed-in on substantially all sides between the three-dimensional form-locking connections.
In accordance with another mode of the invention, two steel layers or sheets are placed on one another as the first and second metallic structure, and at least one of the layers or sheets has an inner surface, in particular an aluminum-containing coating, which can be converted into an insulating layer; the two layers or sheets are first corrugated jointly in a first direction, and are then corrugated once again in a second direction, approximately perpendicular to the first, with major deformation of the first corrugation taking place, creating regular three-dimensional form-locking connections between the layers or sheets; and finally, the convertible inner surface is converted by a chemical treatment and/or a heat treatment into a ceramic insulating layer.
The corrugation of metal layers or sheets can be accomplished by intermeshing gear wheels. The successively accomplished corrugation in two directions being approximately at right angles to one another produces very specific fold forms, which result in highly stable three-dimensional form-locking connections when two layers or sheets are stacked on one another. An aluminum layer on the inside of one of the layers or sheets is readily deformed along with it in the two corrugation operations, and afterward as well is still located everywhere between the two layers or sheets. If that layer is then oxidized, which can be done, for instance, by a heat treatment in oxygen, then a uniform insulating film, which is pressed-in everywhere, between the two layers or sheets, is the result. If the structure has a very large surface area, then substances that can be activated later and contain oxygen can be provided as a coating between the layers or sheets, in order to furnish the oxygen required for the oxidation.
The special field of application of the invention is the stabilization of an electrically heatable catalytic converter, of the kind described in International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711. Those documents are hereby incorporated in their entirety by reference, in order to avoid repetition. In the presence of alternating thermal strains, and when an electrically heatable catalytic converter of that kind is installed in motor vehicles, considerable mechanical strains as well as vibrations occur between the individual parts. Insulation provided by air gaps is therefore not adequate for all fields of use. Instead, it is important to mechanically fix the individual components with respect to one another, so that they can neither approach one another (which would cause a short circuit) nor move away from one another (which would lead to vibration or structural breakage). The supporting structure of the invention is especially suitable for that purpose, because the first metallic structure can be secured metallically to first components, and the second metallic structure can be secured metallically to second components being electrically separated from them. Both generally flat supporting structures, and one or more linear supporting structures, may be used.
As will be described in further detail in conjunction with the drawing, electrically heatable catalytic converters are often constructed as electrically conductive honeycomb bodies and are subdivided into various partial regions. The supporting structure according to the invention can be used simultaneously for purposes of electrical subdivision and mutual fixation of the various partial regions, because it is disposed in the interior of the honeycomb body. The first metallic structure of the supporting structure is metallically firmly bonded to one of the partial regions, and the second metallic structure is metallically firmly bonded to another of the partial regions. This can especially advantageously be performed by hard soldering or brazing, since such honeycomb bodies must often be brazed anyway. However, welding or sintering is also possible as a joining technique. Another option in the use of a supporting structure according to the invention in such honeycomb bodies is to place them on the end surfaces of the catalytic converter and to support various partial regions against one another, in such a way that the first metallic structure of the supporting structure is metallically firmly joined to one of the partial regions, and the second metallic structure is metallically firmly joined to another of the partial regions. In that case, the supporting structure joins various partial regions together mechanically, in the manner of a hoop or a clamp, but not electrically.
Therefore, with the objects of the invention in view, there is furthermore provided an electrically heatable catalytic converter, comprising an interior, end surfaces and first and second electrically conductive and electrically separated components; and an electrically insulating, stabilizing supporting structure being mechanically loadable in all directions for metallic bonding to the components, the supporting structure being disposed in the interior or at the end surfaces and including a first metallic structure, a second metallic structure and an electrically insulating, ceramic material being disposed between the first and second metallic structures and being three-dimensionally pressed-in from substantially all sides, the first and second metallic structures being shaped to exert pressure from substantially all sides onto the ceramic material, without touching one another, and the first metallic structure and the second metallic structure being respectively secured to the first and second components for fixing the components relative to one another.
In accordance with another feature of the invention, the components are part of an electrically conductive honeycomb body having an interior and having an electrical subdivision into various partial regions; the supporting structure is disposed between the partial regions in the interior of the honeycomb body for electrical subdivision and for mutual fixation of the various partial regions; and the first metallic structure of the supporting structure is metallically firmly joined to one of the partial regions, and the second metallic structure is metallically firmly joined to another of the partial regions, in particular by hard soldering, brazing, welding or sintering.
In accordance with a concomitant feature of the invention, the components are part of an electrically conductive honey comb body having end surfaces and having an electrical subdivision into various partial regions; the supporting structure is disposed on the end surfaces of the honeycomb body and supports the various partial regions against one another; and the first metallic structure of the supporting structure is metallically firmly joined to one of the partial regions, and the second metallic structure is metallically firmly joined to another of the partial regions, in particular by hard soldering, brazing, welding or sintering.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electrically heatable catalytic converter and an electrically conductive honeycomb body using the same, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.